- The disorder in which a person cannot see nearby or distant objects clearly is called defect of vision.
- There are two most common defects of vision. They are:
- It is the defect of vision in which a person can see nearby objects clearly but cannot see distant objects.
- It is also called short sightedness or near sightedness.
- For such an eye, the far point is less than infinity.
- Due to the shortening of the focal length of the eye lens or high converging power of the eye lens.
- Due to the elongation of the eye balls.
- In this case the ciliary muscles attached to the eye lens do not relax sufficiently to make the eye lens thinner to reduce the converging power.
- So, the image of a distant object is formed in front of the retina due to the great converging power of the eye lens.
- As a result, the person cannot see distant objects clearly.
- Myopia can be corrected by using concave lens of suitable focal length.
- When a concave lens of a suitable power is placed in front of the eye having myopia, the parallel rays of light coming from the distant object are first diverged by the lens.
- Due to the combined action of both lenses, the image of the distant object is formed at the retina.
- As a result, the person can see the distant objects clearly.
- It is the defect of vision in which a person can see distant objects clearly but cannot see nearby objects.
- It is also called long sightedness, hyperopia or far sightedness.
- For such an eye, the near point is more than 25 cm.
- Due to increase in the focal length of the eye lens or low converging power of the eye lens.
- Due to the contraction of the eyeball.
- It can be corrected by using a convex lens of a suitable focal length.
- When a convex lens of suitable focal length is placed infront of the eye, the rays of the light coming from a nearby object are first converged by the convex lens and then by the eye lens.
- Due to the combined converging action of both the lenses, the image is formed at the retina.
- As a result, the person can see nearby objects clearly.
- This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it.
- This is usually caused by a non-uniform curvature of the cornea.
- A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred.
- Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.
Defects of vision